The invention relates to a pickup tube having a target and an electron gun including a focus electrode and a mesh assembly.
U.S. Pat. No. 4,196,369, issued on Apr. 1, 1980, to Geus et al., discloses a camera tube with a conductive or secondary emission region along a portion of the tube envelope between the input window seal and the mesh electrode to stabilize the potential therebetween. The Geus et al. patent further discloses that electrons, which are dispersed by the mesh electrode or by secondary electrons generated on the mesh electrode, electrically charge the glass wall of the camera tube near the transition between the entrance window and the cylindrical glass tube comprising the sidewall of the tube envelope. The charging creates a picture disturbance which forms a pattern of, for example, annular regions containing more or less signal. The disturbance is especially noticeable at comparatively low light level.
While the Geus et al. patent addresses the problem caused by electrons from the mesh electrode striking and charging the envelope wall in the region between the input window seal and the mesh electrode, it does not address the problem of reducing shading caused by the electrons which do not have sufficient energy to land on the target and are therefore reflected back toward the cathode of the electron gun. The reflected electrons from the periphery of the target tend to land on the mesh retaining ring rather than pass through the mesh electrode into the focus electrode. These reflected electrons produce a circular dark band of shading around the outer edges of the scanned raster. This shading is most apparent when the electron beam current of the tube is set to discharge high signals, and the light level is very low, thus, resulting in the maximum number of return-beam electrons.